Glass annealing is a process of slowly heating up and slowly cooling glass to relieve internal stresses after the glass is formed. Annealing of glass through slow cooling provides a homogeneous structure by reducing internal stresses to give isotropic properties. Glass which has not been annealed is prone to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to its durability. If glass is not annealed, it will retain many of the thermal stresses caused by grinding, polishing and will possess a low overall strength.
During a glass annealing process, the glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at which the glass is still too hard to deform, but is soft enough for the internal stresses to relax. The glass is then allowed to heat-soak until its temperature is even throughout.
In the context of magnetic recording media manufacturing, a glass substrate in the form of a disk needs to be annealed before depositing various magnetic layers thereon. A traditional approach to performing such a glass annealing process on glass disks is by a “disk stacking annealing” process in which a stack 120 of glass disks 122, 124, 126, 128 are provided on a thick glass base 110 as depicted in FIG. 1 before heat is applied to the stack. In this arrangement, both top and bottom surfaces of the glass disks 122, 124, 126, 128 are in contact with and receive heat from other glass disks in the stack 120 during the annealing process.
It has been found that such disk stacking annealing process results in defects such as diffusion, curvy scratches, ID size variations and low disk strength.